Recent evidence suggests that mutations in tissue stem cells generate cancer stem cells, which initiate the formation of many epithelial malignancies. These cancer stem cells are more resistant to chemotherapy and radiation, implying that tumors with a poor prognosis, such as pancreatic ductal adenocarcinoma (PDAC), may originate from cancer stem cells. There is indeed suggestive evidence that PDAC arises from a putative stem cell compartment in the tips of the ducts (the centroacinar cells), but lack of a marker for this cell compartment has precluded direct demonstration of a ductal origin of PDAC. This gap has recently been closed with our identification of the transcription factor Sox9 as an exclusive marker of ductal and centroacinar cells in the adult pancreas. To specifically target this cell compartment, our laboratory has recently developed Sox9-CreERT2 mice that express an inducible form of Cre-recombinase in Sox9-positive cells. As activating mutations in K-ras are common early mutations in human PDAC and initiate formation of PDAC precursor lesions (PanlNs) in mice, I propose to use Sox9-CreERT2 mice to initiate K-ras activation, specifically in ductal/centroacinar cells. This will determine if PanlNs originate from the ductal/centroacinar cell compartment. Based on our recent finding that Sox9 is required for the proliferation and survival of stem/progenitor cells in the embryonic pancreas and is also highly expressed in PanlNs, my second aim will assess a possible role for Sox9 in tumor progression. I propose to test whether lentiviral shRNA-mediated Sox9 inactivation in pancreatic cancer cell lines inhibits cell proliferation and tumor growth in nude mice. I will also test whether Sox9 is required for initiation of K-ras-mediated PanIN formation. These studies will determine whether Sox9-positive ductal/centroacinar cells are the cell-of-origin for PDAC and whether Sox9 promotes tumor initiation and progression. Identification of the cellular origin of PDAC is imperative for the development of effective therapies against resistant cell populations in this highly lethal cancer.